1. Field of the Invention
The present invention relates to two-line adapters for connecting two telephone subscribers' lines together, for performing various tests, placing the lines on hold, and for connecting the lines together and a telephone test set in a conference call configuration.
2. Description of the Prior Art
FIG. 1 illustrates a two-line adapter which is known in the art. The two-line adapter illustrated therein is useful for performing diverse functions including the placing of each subscriber's lines on a hold and the establishment of a three-way conference call between the two subscribers' lines and the telephone test set.
The prior art two-line adapter of FIG. 1 has two pairs of leads 20 and 22 which are adapted to be coupled to the tip and ring lines respectively of two subscribers' lines (not illustrated). The leads 20 are coupled to a protection circuit 24. The leads 22 are coupled to a protection circuit 26. The protection circuits 24 and 26 are of identical construction and function to protect the test set against damage due to high currents which can occur in a fault condition. To place the subscriber's line 20 on a hold, a two pole triple throw switch 28 is positioned to the position labelled "HOLD". To place a subscriber's line 22 on hold, a two pole triple throw switch 30 is positioned to the position labelled "HOLD". In the "HOLD" position, each respective tip-ring pair, 20 and 22, via respective protection circuits 24 and 26, is connected to a pair of oppositely poled Zener diodes 32 and 34 which provide an impedance that is interpreted by the telephone company's central office as representing the closed loop of a telephone call. A telephone test set 36 is connected to the "ON" outputs of the switches 28 and 30. A polarity indicator 38, which indicates the direction of current flow between a single tip and ring line that is comprised of a pair of oppositely poled light emitting diodes 40 and 42, is connected between the terminals labelled "ON" of the two pole triple throw switches 28 and 30 and the pole of switch 44. When the pole of switch 44 is connected to terminal 46, a polarity test is performed. Tone generator 48 is provided to permit the telephone test set to inject tone onto a particular telephone subscriber line 22. The pole of switch 50 is connected to terminal 51 to conduct the tone injection.
With the circuit illustrated in FIG. 1, in order to set up a conference call between the telephone test set 36 and the subscriber's lines 20 and 22, it is necessary to check the polarity of the connections of the pair of leads 20 and 22 to each of the telephone subscriber's lines. The reason for the polarity check is that the battery of the central office must be connected to each subscriber's line with the same polarity so that the polarities of the tip and ring lines of the two subscribers' lines are not reversed. If a reversal of the polarities of the two subscribers' lines occurs, the telephone company central office may interpret one or both lines as being on hook which will cause the line connection to the central office to be lost for the placing of the conference call. Even if the connections are not lost, the telephone test set 36 will not receive sufficient voltage from the central office to permit it to be part of a conference call. Thus, with the circuit illustrated in FIG. 1, it is necessary to sequentially position the two pole triple throw switches 28 and 30 to have their upper poles in contact with the terminal labelled "ON" to conduct the requisite polarity tests. Because of the necessity to sequentially change the upper poles of the switches 28 and 30 to contact the "ON" terminal, there is no continuous indication of the polarity of the connections of the respective pairs of leads 20 and 22 to the subscriber's lines which is useful to linepersons. Because linepersons using telephone test sets often have their hands full, it is undesirable to sequentially have to position the switches 28 and 30 in the manner described above. Moreover, if an error is made by having the polarities of the pairs of lines 20 and 22 coupled oppositely to each other to the tip and ring lines of the respective subscriber's telephone lines, it may be impossible to set up the conference call because of the fact that when the attempt is made to place the second line on hold by calling through to the central office either the first, second, or both line connections with the central office may be lost and the telephone test set 36 cannot participate in the conference call because of an insufficient voltage. This will necessitate that the lineperson will have to reconnect the polarity of one of the two pairs of lines 20 and 22 in the opposite manner and to further have to go through the whole process of redialing the particular subscriber's lines through the central office, and place each line sequentially on hold to establish the three-way conference call involving the subscriber's lines and the telephone test set 36. Thus, the circuit of FIG. 1 requires sequential movement of the upper poles of the two pole triple throw switches 28 and 30 to conduct the requisite polarity test for placing a conference call and further does not provide a continuous polarity indication of a particular subscriber's lines.
U.S. Pat. No. 4,490,583 discloses a plural line telephone controller which permits plural subscribers' telephone lines to be placed on hold and connected to other subscribers' telephones.